A traction control system for an automotive vehicle may control driving force of the vehicle traction wheels by varying engine torque output or by varying wheel brake pressure. It typically includes a microprocessor that responds to information obtained from wheel speed sensors, a throttle position sensor, and brake actuators. In the case of a vehicle having rear traction wheels and non-driving, steerable front wheels, information on the slip of rear traction wheels is obtained as the speeds of the front and rear wheels are compared.
Although the embodiment described in this application is a rear-wheel-drive vehicle with non-driving front wheels, it will be apparent that our invention will be applicable also to other driveline configurations including front-wheel drive (FWD) vehicles.
In the case of traction control systems using pressure-operated brakes, the microprocessor uses the sensor data to send appropriate signals to the brake actuators to establish the brake control required for the instantaneous vehicle speed, road surface condition and throttle setting. The actuators are designed to suppress wheel slippage by braking the driving wheels in response to commands from the processor. In this way, percentage slip is maintained at an optimum level to maintain the desired traction and lateral stability.
It also is well known in the art to provide antilock brake control to avoid wheel skidding and to maintain vehicle lateral stability while reducing stopping distance. Antilock brake control will effect a high frequency locking and unlocking of the wheels to maintain a desired effective tire/road surface slip ratio. The coefficient of friction between the road surface and the wheel tires varies according to the condition of the road and the characteristics of the tire as well as other factors. The coefficient of friction increases in a generally proportional relationship to the slip ratio up to a point just before the slip coefficient reaches its maximum value. At that point, the relationship between the two variables reverses and the friction coefficient gradually decreases as slip ratio increases.
The lateral force acting on the tires also changes in accordance with slip ratio. The lateral force on the tires is a maximum at zero slip and reaches near-zero when the slip ratio reaches 100 percent. The maximum values for lateral friction coefficient and the coefficient of friction during braking do not occur simultaneously. If, however, brake control or traction control is initiated when the slip ratio is at a value that results in slip coefficient of friction near the maximum value, an adequate compromise is reached that provides sufficient, although not maximum, lateral tire force.